Various types of bulk materials are often stored in large structures until it is necessary to use them. This may include, for example, bulk materials which are fibrous, granular, or finely divided materials. Common examples of such materials are silage, wood chips, coal, grain, and the like. The storage structures are generally cylindrical and may be of various diameters and heights.
When it is desired to remove some amount of the material from such storage structures, it is normally not possible to simply open a door at the bottom of the structure in order to let the material flow out. Fibrous materials, for example, will not flow. Finely divided or granular materials often will not flow properly if they become wet, frozen, or packed so tightly that they are bound together.
Although the bulk material could instead be removed from the storage structure from the top side, this would require that the structure have a series of doors or other such access openings at various heights in the wall of the structure. This not only adds to the expense of the structure but it also increases the likelihood that undesirable air leakage into the structure will result. This would also require that the unloading apparatus be adapted to deliver bulk material out one of the openings in the wall irrespective of the level of bulk material in the structure.
The use of bottom unloading devices is very desirable because the storage structure may be kept sealed, if necessary. Furthermore, there is no need to utilize a series of vertically spaced doors in the wall of the storage structure.
Various types of bottom unloading devices have been proposed for use in unloading silos and grain bins. These unloading devices typically include a sweep arm which pivots around a central opening in the floor of the structure. The arm conveys bulk material to the central opening where other conveyance means transports the material beyond the outer wall of the storage structure.
Some of these prior devices utilize a circular track on the floor of the storage structure to guide and, in some designs, to actually drive the outer end of the sweep arm around the floor. In some of these designs the track is located at a point between the center and the wall of the structure. See, for example, U.S. Pat. Nos. 1,275,558; 2,914,198; 3,289,862; and 3,367,519. In other designs the circular track is disposed within a recess in the wall. See, for example, U.S. Pat. Nos. 3,282,446; 3,298,543; 3,121,501; and Reissue Pat. No. 25,863. In still another design the circular track is adjacent the wall of the structure. See U.S. Pat. No. 3,237,788.
There are many disadvantages associated with these former designs. For example, special installation of the equipment is required. The circular track can only be installed when the storage structure is empty. Where the design requires a track recessed in the wall of the structure, it is necessary for the structure to be specially designed in order to accommodate the apparatus. As a practical matter, apparatus of such design would be used only in storage structures which had been built so as to specially accommodate such apparatus.
Where the design relies upon a separate motor inside the storage structure to drive the outer end of the sweep arm, additional complications are presented. Not only is there the greater risk of equipment failure owing to the presence of a second motor, there is also additional drive equipment within the storage structure which must be operating properly in order to produce the desired result.
In U.S. Pat. No. 2,969,156 another type of bottom unloading device is disclosed. This device also requires a specially designed storage structure having a truncated cone shaped bottom. The outer end of the sweep arm is connected to, and driven around the floor by, a carriage under the floor of the storage structure. Thus, for many reasons, this type of unloading apparatus has very limited application.
In U.S. Pat. No. 2,711,814 there is described apparatus which can be placed inside a grain bin after most of the grain has been removed through a central opening in the floor and the grain has reached an angle of repose between the floor and the side walls. The entire apparatus, including the drive motor, must be inserted into the bin from the top. This apparatus is not designed for use in unloading storage structures which are full, nor is it designed for use with bulk materials other than free flowing materials.
In U.S. Pat. No. 2,934,224 there is described another type of unloading apparatus for use in unloading grain from a bin. This apparatus uses a drag brake to control the pivoting of the sweep auger.
Other types of bottom unloading devices are described in U.S. Pat. Nos. 2,675,931 and 2,635,770. These designs utilize an endless chain having teeth to dislodge bulk material and convey it out of the silo. The large number of articulating joints in the chain result in a need for additional maintenance and frequent adjustments to compensate for chain wear and resultant change in chain length. Furthermore, the drive system for propelling the sweep arm is limited as to the maximum available torque and may not be adequate for large diameter storage structures (i.e., over about 21 feet in diameter). Moreover, the space available for appropriate gearing at the inner end of the sweep arm (at the center of the storage structure) inherently limits the horsepower that may be applied to the chain.
Other disadvantages are apparent in conventional chain-type unloaders. Because of the inherent flexibility of the chain, the cutters or teeth on the chain have a tendency to drag on the floor of the storage structure where the chain is at its lowest point. For this reason nearly all floors in storage structures accommodating chain-type unloaders are completely covered with a layer of steel sheets welded together at their edges. Continual wear on the sheets by the chain will eventually cause some of these welds to deteriorate, whereupon the edges of some sheets will buckle upwardly and be caught by the teeth on the chain. This is damaging, both to the teeth and the steel sheets, and in many cases requires that repairs to the floor be undertaken before further material can be removed from the storage structure. This type of repair, of course, is expensive and hazardous since it must be performed while a column of bulk material is still in the storage structure. Unfortunately, this is a rather common occurrence.
Still another disadvantage of chain-type unloaders is that they have a tendency to circulate air between the outside of the storage structure and the inside thereof, thus causing an undesirable increase in oxygen content within the structure and increasing the tendency of the bulk material to spoil.
Another problem associated with many of the prior designs for bottom unloading devices is that the sweep arm pivots at a constant rate. At times the sweep arm loosens bulk material faster than the discharge conveyor mechanism can remove the material from the storage structure. U.S. Pat. No. 3,298,543 describes one manner of alleviating this problem by providing control means which interrupts the pivoting of the sweep arm when the load on the discharge conveyor exceeds a given level.
Since the late 1940's a very large number of farm silos have been constructed using the bottom unloading principle. This market has been dominated by a single company which has constructed the vast majority of all such silos. When intended for the storage of forage materials, nearly all such silos have been built with a trough in the floor which is 7.5 inches deep and 24 inches wide and is intended to accept a chain type unloader manufactured by the same company. Tens of thousands of these unloaders are in current service and many of them reach the end of their useful life and require replacement each year.
Although many attempts have been made to devise an auger-type unloader which would fit directly into the trough of the type of silos described above, the dimensional limitations imposed by the trough size have defeated all such attempts prior to the present invention. The sweep auger requires a rather low rotational speed (under 100 revolutions per minute) with exceedingly high torque capability. Some types of forage materials (especially alfalfa haylage and the like) can be quite difficult to move. Consequently, the central area where the forage moves from the floor level into the trough (where it is moved by the discharge conveyor) must be kept as open as possible and free from impediments and obstructions which would interfere with free and ready movement of the material.
The apparatus and techniques of the present invention overcome these problems.